The mini’s start sequence is critical to it’s ability to print, it’s what tells the printer how to heat up, clean the nozzle and find the plane of the bed before the print. It’s written in g-code, which is a set of movement commands (“G”) and modifier commands ("M), which change things like acceleration and temperature. Here’s an explanation of what the start gcode actually does in case you want to modify it for new filaments, or are just wondering what it’s actually doing between when you click print and when it starts.

*First, it starts heating the bed and nozzle to the temperatures that you’ve set in the print configuration. It also sends the printer to the top front left so it knows what position it’s in.

“G92 E0 ; set extruder position to 0
M140 S{print_bed_temperature}; get bed heating up
G28 ; home all”

*Once it reaches the lowest temperature that the filament can be removed at, it retracts the filament out of the heated part of the nozzle so it doesn’t start to drip during the bed probing. If you’re printing with a new material, you can find this temperature by slowly heating up the nozzle with the filament cooled in the hotend, and chose the lowest temperature where you can easily pull the filament out.

*Next, it heats the nozzle to a temperature where the plastic can be wiped off, but won’t flow out of the nozzle. The printer moves down and wipes the nozzle on the pad behind the bed, you can change the movement commands in this section if you find a material that needs a different kind of wipe sequence.

I’m using lulzbot orange PLA from Village Plastics. Sometimes I still get a bogus probe reading on X0, Y0 the first time ( often as much as .79mm too low ). If I add a couple more G29 commands, the last 2 readings are the different than the first reading but consistent with each other. I’ve seen -1.93 then -1.14 and -1.14 for the second and third probe of the same point. It seems to me that the last 2 readings are more representative of the actual bed height.

What do you think? Is there any harm of adding extra G29 commands in start.gcode other than the time it takes to complete the probe?

You’ll want to have your start/end gcode updated with a better cleaning temperature if you’re not using our pre-tested filament from LulzBot. The cleaning temperature needs to be high enough to allow the filament to be wiped off the nozzle, but not hot enough to cause the filament to drip out of the extruder nozzle.